I think you only need a model release (or property release) if you're using that model (or property) for financial or commercial gain.
Otherwise you'd need a release to show the image to anyone to ask what they thought of it.

There are actually some really good posts on these forums about when you do and don't need a model release. For the most part as it boils down you only need a model release if the person is identifiable and the main subject (ie crowds don't count) and that you are somehow associating the person with an idea/concept. In other words you need a release if you are say taking a picture of baseball catcher for a sign for a local park since it makes the assumption that the person is endorsing that park.

In the UK you don't "need" a model release ever... however, some companies won't buy or use your model shots for commercial usage without one, so if possible, get one just in case. On TPF, as long as the content is safe then there's no problem.